Living Roof

The urban environment is an ecosystem in itself. With this living roof, BRIT is bringing a functional, native Texas ecosystem back into the built environment. Rather than just looking for plants that can survive in hot environments, BRIT asked the question, “What are the environmental parameters of a roof and what are its analog and native environs?"

Designed to mimic a geology formation known as the Goodland/Walnut Barrens, BRIT's roof represents possibly the largest Texas barrens habitat of its type and is one of the only living roofs in Texas modeled after a true native ecosystem. Prairie barrens ecosystems are characterized by extremely shallow, limestone soils and low water throughout most of the year, perfect conditions for a roof in Fort Worth.

Most importantly, our living roof creates a useable ecosystem in space that would otherwise go unused by the biological community. When it's hot, the living roof also maintains a lower daytime temperature than the non-living roof, reducing heat island effect and insulating the inside of the building. During the winter, the roof insulates the building from the cold. The rainfall collection system from the living roof allows for water to be reused for irrigation on the campus and helps to mitigate storm water surges during rain events.

Design and Construction

BRIT researchers, along with associates at Texas Christian University, began work on the project in 2007. As the compositions of these ecosystems were described, planting ideas were drawn up for the living roof, as well as design specifications for the planting medium. Similar to the prairie soil, the characteristics of the soil to be used on the roof needed to be defined and matched to topsoil with specific microbes and seed bank. A matching topsoil was found and transplanted to the roof from Little Bear Aggregate site in Cresson, TX.

BRIT's roof sits at a 9.5 degree angle to the south, mostly to facilitate viewing from ground-level but also to promote drainage. Biodegradable planting trays were used to prevent erosion during establishment and allowed for a modular design and easy installment. Each 2’ x 2’ coconut fiber tray was planted with six native Texas species (from a list of 38 total test species) within just 3 inches of mixed native and engineered soil with a limestone gravel mulch layer on top. The trays were started on the ground in late July 2010, tended in a sheltered spot under the tree canopy on the east side of the parking lot, and delivered onto the roof in November 2010 using a conveyor belt.

Design specifications:

This 20,850-s.f. North Texas research site is located on top of a two-story building at the Botanical Research Institute of Texas. The overall roof design mimics a natural, thin-soiled, limestone prairie habitat that requires minimal inputs once established. The modular roof system is composed of nearly 5700 biodegradable coconut fiber trays (BioTray, Tremco Roofing). In late July 2010, each 0.19 m2 (2-s.f.) tray was planted with six native Texas species (five 10-cm transplants and a sixth annual species from seed) from a list of 38 total test species (Table 1). Test species were chosen based on their adaptation to thin, dry soils and their nativity to a specific local habitat, the Goodland Limestone Prairie Barrens. The trays held approximately 7.5 cm of growing media with another 5 cm of media below the trays. Media beneath the trays consisted of 1:1 calcareous sandy loam topsoil (CSL) and commercial aggregate media (LiteTop mix; American Hydrotech, Inc.). Media within the trays consisted of a lower 3.8-cm layer of 1:1 CSL and hadite; an upper 2.5-cm layer of 1:1:2 CSL, hadite, and biologically active Goodland Limestone topsoil harvested from a local prairie; and a 1.2-cm gravel mulch layer on top. Layered between the media and monolithic roof membrane were a fine mesh filter fabric, an aggregate-filled drainage layer (Gardendrain GR30 and LiteTop Aggregate; American Hydrotech, Inc.), polystyrene foam insulation, a drainage mat (Hydrodrain 300, American Hydrotech, Inc.), and a copper-based root protection sheet (Hydroflex RBII, American Hydrotech, Inc.).

The drainage system was designed to retain 7.6 l/m2 with a flow rate of 479 l/min/m. One hundred percent non-potable irrigation was achieved by using harvested stormwater and an on-site underground spring. In addition to natural rainfall, all plants received approximately 15.7 mm/wk supplemental, dry-season irrigation (May–Sept) during the first 12 months of establishment (Aug 2010–Aug 2011), with additional irrigation during extreme drought conditions in June–July 2011. At the end of the growing season, irrigation tapered to 7.85mm/wk for two weeks and then ceased completely until the beginning of the following dry season (May 2012) when occasional, minimal irrigation was provided as needed (e.g. 7.85 mm every 2–4 weeks). Plant survival was assessed 18–24 months after initial planting during March–July 2012 using centralized replicate sampling where twenty-seven 0.25 m2 roof plots were subjectively chosen within homogenous, predetermined planting areas. Every species in each of these plots was identified and documented.

Layers of the roof:

Research

Weather Data

A weather station was installed on the roof in 2011 to collect data on rainfall, temperature, humidity, wind speed and direction, UV, solar radiation, evapotranspiration, and air pressure. To compare temperature and humidity, two more satellite stations were set up, one in the middle of the prairie and one on the northeast corner of the non-vegetated solar roof of the herbarium building. Data are collected every five minutes from all weather stations. Visit bdi.brit.org/weather(temporarily down for maintenance) for real-time weather conditions on BRIT's campus. For historical weather data back to 2011 (living roof only), visit BRIT's page at Weather Underground.

Vegetation Surveys

After the initial roof installation, observational vegetation surveys were done on a monthly schedule. Attention was paid to any invasive species or weeds that were coming up on the roof, as well as species of plants that were not on the original planting list. Formal vegetation surveys were started in the second year, analyzing species diversity relative to initial planting scheme (i.e., one of three vegetation assemblages) and slope position on the roof (high versus low). Overall success of the original planting palette was reported in a paper published in 2013 in Journal of Living Architecture and in a chapter within the 2015 book Green Roof Ecosystems.

Arthropod Surveys

Arthropod surveys of the roof were conducted from July through November of 2012 under the direction of Dr. Brooke Byerley Best and BRIT intern Adam Ulissey, a student at El Centro College. These surveys utilized pitfall traps to document and inventory ground-dwelling insects on both the living roof and the ground-level prairie. Undergraduate Sydney Jackson from Austin College assisted with processing and identifying samples from both the roof and ground sites.

Variation in Arthropod Community on a Prairie-Style Green Roof Relative to Vegetation and Slope Position -- upcoming 2019 conference presentation by Sydney Jackson on final roof data (Texas Academy of Science, Brownwood, TX, March 2019)

A side-project performed by TCU students Haley Rylander and Devon Spencer involved collecting and identifying insects from a reference prairie site at Dutch Branch Park. These data are still being processed but should inform BRIT researchers as to the soil health of the roof in comparison to an intact prairie as well as help guide other living roof researchers on the merits of native soil inoculation with respect to soil biota establishment.